Low-resistance grounding member

ABSTRACT

A threaded fastening and grounding member having an upward projecting lip for frictionally engaging a threaded recess in a flush-mounting box to provide a low-resistance electricalgrounding connection between the fastening member and the threaded recess.

United States Patent William A. Muslin 1378 Ryan. Roseville. Minn. 55113;

James C. Schacker, 1504 Edgewater, Arden Hills, Minn.; Earl E. Mcl-lattie, 2129 Midlothian Road, lRuseville, Minn. 55113 Appl. No. 816,666

Filed Apr. 16, 1969 Patented July 20, 1971 Inventors LOW-RESISTANCE GROUNDING MEMBE 7 Claims, 4 Drawing Figs. 1

US. Cl 339/14 R, 151/22 R int. Cl lHOlr 3/06 Field 01' Search. 339/14; 151/22 H 1 359mm [56] References Cited UNITED STATES PATENTS 226,450 4/1880 lbbotson et al. 151/22 1,764,168 6/1930 Hosking H 151/22 X 1,933,332 10/1933 May 85/47 2,067,796 1/1937 Smith 339/14 Primary Examiner-lan A. Calvert Assistant Examiner-Robert A. Hafer A1t0rneyStryker and Jacobson ABSTRACT: A threaded fastening and grounding member having an upward projecting lip for frictionally engaging a threaded recess in a flush-mounting box to provide a low-resistance electrical-grounding connection between the fastening member and the threaded recess.

LOW-RESISTANCE GROUNDING MEMBER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to the field of electrical connectors and, more specifically, to means for forming a low-resistance electrical-grounding connection between a threaded recess in a metal flush box and a fastening member that fits into a threaded recess to hold an electrical outlet in the metal flush box.

2. Description of the Prior Art In order to prevent or minimize the hazards of receiving an electrical shock from a faulty appliance, conventional wiring circuits are provided with a low-resistance electrical-grounding path from the wall outlet receptacle to an earth ground. Such a grounding path protects the operator of a piece of electrical equipment against dangerous electrical shorts that might occur in equipment wiring. The equipment, such as household appliances, have a 3-wire cord connected to a 3-prong male plug. Two of the wires are there for the usual purposes of supplying power to the appliance. The third wire is connected to the exposed metal parts of the appliance so that in the event the appliance should become electrically hot" because of a wiring fault or the like, a low-resistance grounding circuit is provided to remove any dangerous potential from the exposed parts of the appliance. In this event, a resultant momentary high current will ordinarily blow a fuse or other protective device thus removing the power from the unit. Even though removing the power makes the equipment inoperable, it removes the danger of an electrical shock to the operator. The receptacle in which a 3-prong plug is ordinarily inserted is usually mounted in the wall in a metal flush box which is electrically grounded through a metallic conduit to an earth ground.

One of the problems involved in grounding the outlet receptacle to the earth ground is the forming of the low-resistance electrical connection between the wall outlet receptacle and the grounding lead, which in this case, is the metallic conduit.

' One of the prior art grounding methods employed a wire lead from the wall outlet-grounding receptacle to the metal box in a similar manner as the two powered inlets are wired. While providing an adequate ground connection this method has been found to be rather expensive and time consumingas a short piece of wire must be cut and formed to fit onto the outlet and onto the flush-mounting box. Also, this extra ground wire further fills up the relatively small flush-mounting wall box.

An improved means that eliminates the grounding wire by forming a grounding path from the wall outlet receptacle to the fastening member that holds the outlet receptacle to the wall is shown in patent application Ser. No. 594,177 filed Nov. 14, 1966, now U.S. Pat. No. 3,432,793. This type of a grounded outlet allows one to quickly ground the outlet through a metal clip located on each end of the outlet receptacle by inserting a threaded fastening member through the metal clips and into the threaded recess in the wall box. These metal clips tightly contact the threaded fastening member to produce a low-resistance electrical connection therebetween. While this type of electrical receptacle provides a low-resistance electrical connection between the fastening member and the outlet receptacle, it does not always ensure a low-resistance electrical grounding path from the fastening member to the wall box. Oftentimes, in the process of building a house, plasterboard or chips of material become lodged behind the outlet receptacle and the wall box. Thus, when a fastening member such as a screw is inserted into the wall box, the operator cannot pull the metal clips on the outlet receptacle into a good electrical connection because of the material between the outlet receptacle and the wall box. Also, the plasterboard or insulating material oftentimes becomes lodged between the threads on the fastening member and the threaded recess in the wall box. These particles produce a high-resistance electrical connection between the fastening member and the wall box. Obviously, this high resistance electrical connection is undesirable because it increases the potential or hazard of an operator's body forming the grounding circuit rather than the desired grounding circuit through the conduit.

In order to produce a low-resistance electrical-grounding connection between the fastening member, such as a screw or the like, and the wall mounted box, the present invention provides a threaded member having an upward extending collapsible lip. This upward extending collapsible lip frictionally engages the threaded recess in a self-cleaning or scouring action so as to produce a metal-to-metal low-resistance electrical connection between the fastening member and the threaded recess in the wall box. Thus it is not necessary that the clip on the outlet receptacle be in firm contact with the wall mounting box to achieve a low-resistance electrical contact because the self-cleaning action of the threaded member as it is screwed into the threaded recess produces a low-resistance electrical connection between the fastening member and the threaded recess. This type of grounding or threaded member has the added advantage in that it does not require any additional wire to ensure that the unit is properly grounded to the wall box. By eliminating the wire and using the threaded member as part of the ground, it prevents the operator or tradesman from inadvertently failing to ground the electrical outlet.

SUMMARY OF THE INVENTION Briefly, the invention comprises a threaded member having an upward extending collapsible lip that projects beyond the major diameter of the screw threads so as to frictionally engage and clean the threaded recess in an electrical wall box to produce a low-resistance electrical ground connection between the wall box and the threaded member.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a side view of the threaded grounding member having an upward extending collapsible lip;

FIG. 2 is a cross-sectional view taken along lines 2-2 of FIG.

FIG. 3 shows partially in section an assembled wall outlet receptacle and a wall mounting box having a chip of building material located therebetween; and

FIG. 4 shows a ram forming the collapsible lip.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, reference numeral 10 generally designates a low-resistance grounding and fastening member suitable for forming a low-resistance ground electrical connection between itself and a wall mounting box. Grounding member comprises a threaded section Ill and a head section 12 having a driving slot 113 for turning threaded member 10 into a suitable threaded recess. Threaded section 111 comprises a first section 1 In of conventional threads and a second section Illb of sheared threads. The first section 11a of conventional threads allows an operator to easily start the fastening member into a threaded recess. The major diameter of threads II II is indicated by reference numeral 16 and the minor diameter of threads is indicated by reference numeral 17. Normally, the major diameter of the threads constitutes the largest dimension of the threads. In the present invention there is provided an upward projecting, collapsible or deformable lip 20 The desirable feature of having a low-torque screw for forming a low-resistance electrical ground becomes apparent when one considers that to fasten the grounding screw one must rotate the grounding screw approximately 18 complete turns. As each turn requires two hand operations, and with two grounding screws per receptacle, it becomes apparent that the effort or torque required to turn the screw is excessive as it negates the advantage of the low-resistance electrical connection. Typical test results have shown that the torque required to turn a conventional nondeformed screw into an outlet receptacle is approximately 1% oz.-inches. However, the resistance between the threads in the receptacle and the screw threads is unacceptable. On the other extreme are the wedged or malformed threads which require a torque of 80 oz.-inches. Although these screws produce a low-resistance electrical ground, they are too difficult for an operator to continuously turn into the receptacle.

Tests have revealed that the present invention requires only a torque of approximately 16 oz.-inches to produce a low-resistance connection between the screw and the receptacle. With torques on the order of 16 oz.-inches the screw can be snugly but easily turned into the threaded recess.

These torques are given for illustrative purposes and no limitation is intended thereto. However, they do indicate the relative force required to turn the various types of screws.

The screw shown in FIG. 2 is a conventional right-hand screw that is advanced into a wall-mounted box by counterclockwise rotation of screw 10, as viewed in FIG. 2. Lip comprises a continuous, smooth leading edge and a sharp trailing edge 20a. This is important if lip 20 is used with soft metals such as aluminum as the sharp edge 20a would gouge the soft metals; however, for harder metals the sharp trailing edge 20a could be the leading edge as it will collapsibly follow a threaded recess in harder material without gouging the threaded recess.

By separating the threads with a minimum of deformation through a shearing action rather than a wedging action, it allows an operator to rather easily force the sheared threads into the threaded recess whereas if the threads had been deformably wedged apart, it would be rather difficult to force the deformed threads into a threaded recess.

Referring to FIG. 3, there is shown a flush mounting wall box 30, screw 10 and a self-grounding electrical receptacle 25. Grounding electrical receptacle is of the type shown and described in the aforementioned patent. Grounding receptacle 25 contains a resilient-grounding clip 25a that provides a low-resistance electrical-grounding connection between clip 25a and member 10. For illustrative purposes, FIG. 3 shows a piece of plaster 31 located between outlet 25 and flushmounting box 30. This piece of plaster or the like prevents electrical receptacle 25 or clips 25a from directly contacting wall mounting box and thereby providing a low-resistance electrical-grounding contact by the pressure contact between fastening member 10 and clip 25a. However, utilizing a threaded member with a collapsible lip 20 provides a low-resistance electrical connection even though the wall-mounting box 30 and grounded outlet 25 are not in actual physical contact.

FIG. 4 shows a ram 40 for forming an upward protecting collapsible lip 20 in fastening member 10. An operator moves ram 40 longitudinally along the axis of fastening member 10. This shears member 10 and forces lip 20 apart from the threaded section 11 of member 10. Normally, ram 40 is moved in a straight line so as to uniformly upset the major diameter of the screw along a line parallel to the central axis of member 10 to thereby produce a straight lip that projects beyond the conventional major diameter of the threaded section. However, it is envisioned that lip 20 need not be straight but could be helical or the like as long as it extends partially along the threaded section of member 10.

We claim:

1. A grounding assembly suitable for forming a low-resistance electrical-groundin connection between a conductmg and fastening means an a second member comprising: a

second member adaptable to be connected with a grounding circuit; conducting and fastening means having a central axis, said conducting and fastening means having a section located thereon for forming interlocking engagement with said second member; said conducting and fastening member having not more than one deformable lip in order to thereby form a lowresistance electrical connection between said conducting and fastening means and said second member, said deformable lip projecting outward from said conducting and fastening means so that movement of said conducting and fastening means causes said deformable lip to frictionally engage said second member to thereby scour the surface of said second member to produce a low-resistance electrical connection between said conducting and fastening means and said second member, said deformable lip further collapsible inwardly in response to pressure between itself and said second member to thereby prevent gouging of said second member during movement of' said deformable lip.

2. The invention as described in claim 1 wherein said section for forming interlocking engagement with the second member comprises a helical-shaped engaging member and said second member comprises an outlet box.

3. The invention as described in claim 2 wherein said helical-engaging member comprises a screw thread.

4. The invention as described in claim 1 wherein said lip projects in the direction opposite the direction of advance of said fastening member.

5. The invention as described in claim 1 wherein said second member comprises a wall-mounting box.

6. The invention as described in claim 1 wherein said second member includes a self-grounding outlet receptacle suitable for forming a low-resistance electrical connection between said outlet receptacle and said fastening means.

7. The invention as described in claim 1 wherein said lip is located longitudinally along'said conducting and fastening means. 

1. A grounding assembly suitable for forming a low-resistance electrical-grounding connection between a conducting and fastening means and a second member comprising: a second member adaptable to be connected with a grounding circuit; conducting and fastening means having a central axis, said conducting and fastening means having a section located thereon for forming interlocking engagement with said second member; said conducting and fastening member having not more than one deformable lip in order to thereby form a low-resistance electrical connection between said conducting and fastening means and said second member, said deformable lip projecting outward from said conducting and fastening means so that movement of said conducting and fastening means causes said deformable lip to frictionally engage said second member to thereby scour the surface of said second member to produce a low-resistance electrical connection between said conducting and fastening means and said second member, said deformable lip further collapsible inwardly in response to pressure between itself and said second member to thereby prevent gouging of said second member during movement of said deformable lip.
 2. The invention as described in claim 1 wherein said section for forming interlocking engagement with the second member comprises a helical-shaped engaging member and said second member comprises an outlet box.
 3. The invention as described in claim 2 wherein said helical-engaging member comprises a screw thread.
 4. The invention as described in claim 1 wherein said lip projects in the direction opposite the direction of advance of said fastening member.
 5. The invention as described in claim 1 wherein said second member comprises a wall-mounting box.
 6. The invention as described in claim 1 wherein said second member includes a self-grounding outlet receptacle suitable for forming a low-resistance electrical connection between said outlet receptacle and said fastening means.
 7. The invention as described in claim 1 wherein said lip is located longitudinally along said conducting and fastening means. 